Lithium ion secondary batteries have high energy density and excellent charge/discharge cycle characteristics, and are thus widely used for a power supply for compact mobile devices such as cellular phones and laptop computers. In addition, the recent increasing environmental considerations and growing consciousness of energy saving have been promoting a demand for large batteries having a large capacity and a long life in the fields of electric vehicles, hybrid electric vehicles, power storage, etc.
In general, a lithium ion secondary battery primarily consists of: a negative electrode including a negative electrode active material of a carbon material capable of intercalating and deintercalating a lithium ion; a positive electrode including a positive electrode active material of a lithium composite oxide capable of intercalating and deintercalating a lithium ion; a separator separating the negative electrode and the positive electrode; and a non-aqueous electrolytic solution prepared by dissolving a lithium salt in a non-aqueous solvent.
Amorphous carbon or graphite is used for the carbon material used as the negative electrode active material, and graphite is typically used particularly in an application which requires a high energy density.
For examples, Patent Literature 1 discloses that in order to obtain a non-aqueous electrolytic solution secondary battery which exhibits a high capacity and a high charge/discharge efficiency, a negative electrode active material is used which includes a carbon material containing at least two materials of a scale-shaped graphite particle and a graphite material the surface of which is covered with amorphous carbon and which is not scale-shaped, the packing density of the negative electrode being in the range of 1.3 to 1.8 g/cc, the specific surface area of the negative electrode being in the range of 2.1 to 4.1 cm2/g, and the fraction of the scale-shaped graphite particle being in the range of 10 to 70 wt % based on the whole of the carbon material.
Patent Literature 2 discloses that in order to obtain a non-aqueous electrolyte battery which has a high capacity and high cycle characteristics and exhibits a high volume energy density even in discharging at a large current, a negative electrode active material is used which includes a negative electrode active material mixture of scale-shaped graphite and at least one or more carbon materials selected from spheroidal graphite, bulk graphite, fibrous graphite, non-graphitizable carbon, and carbon black, the content of the one or more carbon materials in the negative electrode active material mixture being in the range of 1 wt % or more and 50 wt % or less.
Patent Literature 3 discloses that an active material including a mixture of an artificial graphite particle having a tap density of 1 g/cm3 or higher and a spheroidal graphite particle having a large roundness is used for the purpose of significantly improving the charge/discharge cycle characteristics of a high-energy density lithium secondary battery, and simultaneously enhancing or maintaining the discharge rate characteristics, the discharge characteristics at low temperatures, and the heat resistance. Patent Literature 3 also discloses that the fraction of the spheroidal graphite particle based on the whole of the active material is preferably 5 to 45% by mass.
Patent Literature 4 discloses a negative electrode active material for a lithium ion secondary battery, wherein the negative electrode active material is a powder mixture of a graphite powder A which is obtained by mixing a scale-shaped natural graphite and a binder pitch, and subjecting the resultant to molding, calcination for graphitization, and pulverization, and has a tap density of 0.4 to 0.9 g/cm3, D90/D10 of 3.5 to 7.0, and D50 of 10 to 20 μm, and a graphite powder B which is obtained by covering a spheroidized natural graphite with a pitch and calcining the resultant for graphitization, and has a tap density of 0.9 to 1.4 g/cm3, D90/D10 of 2.0 to 3.5, D50 of 10 to 20 μm, and a roundness of 0.91 or higher, and the mixing ratio by weight of the graphite powder A is 20 to 80% and the mixing ratio by weight of the graphite powder B is 20 to 80%. Patent Literature 4 also discloses that the object is to provide a negative electrode active material for a lithium ion secondary battery having a high electrode density, an excellent permeability of an electrolytic solution, a small capacity loss due to charging/discharging, and good cycle characteristics.